Radio receivers are well known. Typically, systems are designed to avoid strong interferors in the received band, or provide sufficient dynamic range in the bandwidth of the receiver to tolerate both the interferor and the signal of interest.
In order to avoid interference, the antenna and/or receiver may be designed to reject signals having particular frequency characteristics. Likewise, transmissions with defined temporal constraints may be filtered. Further, directional antennas or antenna arrays may provide spatial discrimination. These methods work to avoid interference if a signal of interest may be distinguished from an interferor by spatial, temporal or spectral characteristics. On the other hand, in many cases, interferors cannot be so distinguished.
It thus is also possible to remove interference. If the interferor is not coincident with the signal of interest, a filter may be applied to block the interferor. This possible solution requires that the interferor be sufficiently predictable that a filter may be provided, and that the filter be sufficiently precise to avoid distortion or degradation of the signal of interest. Where the interferor and signal of interest overlap, a filter is difficult or impossible to employ.
A second method of removing interferors is to provide active signal cancellation. In this case, a model of the interfering signal is created, which is then subtracted from the received signal. This requires that a sufficiently accurate representation of the interfering signal be created, and precisely matched in amplitude and phase with the actual interferor. Where the interferor is modulated, this technique must tolerate the modulated waveform, or model the modulation in real time.
See, for example, U.S. Pat. Nos. 7,362,257, 7,359,465, 6,574,270, 6,570,909, 6,501,788, 7,349,505, 7,349,504, 7,336,930, 7,333,420, 7,295,597, 7,292,552, and U.S. Patent Applications 20030053526, 20020172173, and 20020051433, each of which are expressly incorporated herein by reference.
Active noise cancellation is employed in various fields, such as audio. As the frequency of the interferor to be canceled increases, the difficulty in matching the phase of the signal also increases. If the phase mismatch is more than 90 degrees, the cancellation system can actually increase the interference Likewise, as the bandwidth of the interferor increases, the difficulty in matching the phase delay across the entire band increases in difficulty. Thus, while interference cancellation has been deployed in various systems, its adoption at radio frequencies, for complex interferors, has been limited. See, e.g., Gardner, W. Agee, B., “Two-stage adaptive noise cancellation for intermittent-signal applications”, IEEE Transactions on Information Theory, 26(6):746-750 (November 1980).